Platinum group metals (platinum, iridium, osmium, palladium, rhodium and ruthenium), gold and rhenium are used in a multitude of ways in various industries, such as automobile, electrical and electronic, dental and medical, petroleum refining and numerous chemical industries. The major primary source of platinum group metals, gold and rhenium is from ores and complex ores frequently containing nonferrous metal sulfide deposits, such as Cu—Ni deposits. An increasingly important source of platinum group metals, gold and rhenium, especially in the United States, is that of secondary sources, particularly scrap of ceramics, glass, electrical and computer components and spent catalysts, e.g., from petroleum refineries and automobile catalytic converters.
For example, about 45 million automobiles are scrapped worldwide every year, including more than 15 million in the United States alone. Many of these automobiles have catalytic converters containing platinum group metals, gold and/or rhenium in sufficient quantities to warrant recovery attempts. For example, approximately 60% of scrap catalytic converters are collected to recover platinum group metals at a recovery value of approximately $35–40 per catalytic converter.
Platinum group metals, gold and rhenium frequently are incorporated with rare earth elements, such as cerium, lanthanum and neodymium, which are imbedded into the catalyst matrix consisting primarily of aluminum and silicon oxides. Effective extraction of these rare earth elements usually facilitates the recovery of platinum group metals from these catalysts.
Because platinum group metals, gold and rhenium are regarded as chemically noble, their extraction from various source materials is relatively very difficult and very expensive. Aqua regia (HCl/HNO3) and concentrated HCl/Cl2 solutions have been used in the precious metals industry to put these metals into solution. However, these reagents are chemically strong. It is very difficult and expensive to safely and efficiently handle these reagents under the concentrations used in the industry. The high acidity concentrations also make reagent recycling exceptionally difficult. The necessary neutralization of acid strength before metal extraction from the solution increases costs and inefficiencies.
Extracting platinum group metals, gold and/or rhenium from automobile catalysts is also relatively difficult and expensive, particularly due to the problems associated with handling the acids employed and the high cost of reagent consumption. The chemicals and methods commonly used to process these metals tend to dissolve even silica and alumina, which frequently make up the base matrix holding the platinum group metals. As a result, existing processes generally suffer from high acid consumption and severe acid corrosion problems.
A non-acidic process of dissolving platinum group metals has been introduced, which appears to be an improvement in metallurgical efficiency in some aspects. However, a major reactant of this non-acidic process is cyanide, a toxic chemical presenting its own handling, processing and disposal issues. This non-acidic process also suffers from relatively high reagent consumption and relatively low recovery of rhodium.
Researchers at the South Dakota School of Mines and Technology have developed certain technologies of extracting precious metals, including gold, silver, copper, nickel, rhenium and platinum group metals from ores and spent catalysts using ammonia and/or halogen salts. See, e.g., U.S. Pat. Nos. 5,114,687; 5,308,381; 5,328,669; and 5,542,957. In general, these processes involve the recovery of precious metals using environmentally benign processes. However, these processes also generally involve higher temperatures and relatively higher pressures, such as in an autoclave.
Another process to recover platinum group metals from various sources involves roasting the source material with one or more of sulfuric acid, a sulfate and/or a bi-sulfate and with one or more halogen salt, and by contacting the roasted product with a leaching solution as disclosed by pending application Ser. No. 10/280,714.
Therefore, a need exists for an improved process of recovering platinum group metals, gold and/or rhenium from a variety of sources.